Radio transponders are well known in the transportation field. For example, an air traffic controller may request that a pilot activate his plane's transponder to return a specified code signal when the transponder receives the controller's radar signal. This enables the air traffic controller to identify which one of the numerous "blips" on his radar screen represents the pilot's aircraft. Similar transponders have been attached to railroad freight cars. These devices are preprogrammed to return the serial number of the freight car to an interrogating station. The transponders aid in locating the cars along the railway system.
More recently radio transponders have been temporarily attached to workpieces, for example to automobiles, on an assembly line. As each piece reaches a workstation, it is identified to the manufacturing equipment located at the station. The manufacturing equipment upon detecting the presence of a specific workpiece, interrogates a central data base for information regarding the features of the piece. This enables different types of workpieces to be intermixed on the same assembly line and to identify which options are to be incorporated in the workpiece presently being worked.
A common feature of all of these transponders is the transmission of a unique code sequence upon receiving an interrogating signal at a particular radio or microwave frequency. Up until recently, this unique code sequence was either permanently programmed into a given transponder or an operator had to establish it by a series of switch settings. In several applications of transponders, it is desirable to periodically change the code sequence. For an assembly line, it is desirable to enable the transponder to transmit data regarding the workpiece, such as its color, to the automated manufacturing equipment. With the permanently programmed device, the code sequence could not be changed without replacing the transponder. Although the other type of device allowed the code sequence for a given transponder to be changed, it required operator intervention.